The present invention relates generally to reducing bending vibrations in a structure, and more particularly to an active vibration control subassembly capable of generating canceling vibrations in the structure to offset such bending vibrations.
Structures which undergo bending vibrations from a source include, but are not limited to, annularly-cylindrical shaped superconductive coils of magnetic resonance imaging (MRI) superconducting magnets, patient positioning tables of X-ray machines, ducts of aircraft engines, and panels of household washing machines. For example, the annularly-cylindrical shaped superconductive coils of a medical MRI superconducting magnet undergo bending vibrations induced by electromagnetic forces generated during magnet operation. The bending vibrations of the superconductive coils cause blurry MRI images which are undesirable since sharp MRI images are needed for accurate medical diagnosis.
Known passive vibration control techniques for reducing bending vibrations in a structure include making the structure thicker, making the structure with stiffening ribs, and adding weights or damping at discrete locations to dampen troublesome vibrations modes. However, adding thickness, ribs, weights, or damping makes the structure more bulky in applications where increasing the weight of the structure is undesirable, such as in aircraft engines.
Known active vibration control techniques for reducing bending vibrations in a structure basically sense the motion of the structure with accelerometers and the like, then calculate the bending vibrations from the sensed motion using a computer or other controller, and then produce canceling bending vibrations generally equal in amplitude and opposite in phase to the calculated bending vibrations. Conventional techniques for generating the canceling bending vibrations include using piezoceramic actuator plates to bendably vibrate the structure to produce the canceling bending vibrations. The piezoceramic plate is driven by an electric AC signal such that when the signal is positive, the plate causes the structure to bendably deflect in a first direction from its resting state, and when the signal is negative, the plate causes the structure to bendably deflect in the opposite direction.
The larger the amplitude of the electric AC signal driving the piezoceramic actuator plate, the larger the canceling bending vibration produced in the attached structure. However, the piezoceramic actuator plate will structurally fail when the applied electric AC signal causes the plate to exceed its critical tensile stress (which is smaller than its critical compressive stress). It is noted that the particular value of the critical tensile (or compressive) stress depends on the particular piezoceramic material being used. What is needed is an improved subassembly, of an active vibration control system, for generating large canceling bending vibrations to reduce the bending vibrations produced in a structure by a source independent of the subassembly.